Field of the Invention:
The present invention relates generally to hydraulic control systems and more particularly to systems for use in automatic transmissions equipped with hydraulic servo-mechanisms and accumulators therefor.
Clutches and brakes equipped with hydraulic servomechanisms, brakes and the like are used in transmissions to establish a given change gear ratio. A hydraulic control circuit is provided which includes a plurality of valves for controlling pressures to be fed to these servo-mechanisms.
However, it has been found that several problems are encountered with prior art hydraulic control circuits. Among the problems encountered are the following:
(1) In circuits equipped with a shift timing valve, even when the shift timing valve has an auxiliary exhaust passage connected thereto, the provision of an accumulator results in an increase in time required from the time of actuation of the shift timing valve until a fluid pressure in the servo-chamber on a release side reaches a given pressure level to release an engaging member. Stated differently, there takes place an overlapped engagement of engaging members both on a high speed side and on a low speed side, thus causing a locking condition and wear of the engaging members.
(2) Even in a case where a diameter of an orifice is so designed as to nullify the overlapping time, a variation in coefficient of viscosity and a variation in diameter of the orifice due to manufacturing error may lead to underlapped or overlapped engagements of engaging members in an automatic transmission.
Thus, difficulty is encountered with the manufacture of automatic transmissions with regard to providing desired performances.
It is a primary object of the present invention to provide a hydraulic control system for use in an automatic transmission, which avoids the aforesaid two problems (1) and (2) and which allows a sharp drop of a fluid pressure in the servo-chamber to be released, the moment fluid pressure on another servo-chamber to cause engagement of another engaging member reaches a given pressure level.